CMake: Add information about the current state of support (#13551)

cmake/README.md

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+## CMake for Mbed OS
+
+Until further notice, CMake for Mbed OS is still in development. 
+
+Application profile selection (`bare metal` or `full`), printf library selection (`minimal-printf` or `std`), C library selection (`small` or `std`) and other features that require altering compiler/linker command line options are not yet supported.
+The full profile with the standard printf and C library are selected by default.
+
+### Supported targets
+
+Only a limited set of targets is supported at the moment.
+
+These targets should work:
+- K64F 
+- DISCO_L475VG_IOT01A 
+- NRF52840_DK 
+- WIO_3G
+- K66F
+
+### Supported toolchains
+
+Arm Compiler 6 and GNU Arm Embedded toolchains are supported.
+
+### Examples
+
+Supported examples can be identified by the presence of a top level `CMakelists.txt` input source file. 
+
+### Known issues
+
+- ARMClang6: the assembler does not include definitions. It will be fixed in the upcoming release (>3.18.2). Reference: https://gitlab.kitware.com/cmake/cmake/-/issues/21148#note_823305
+- Ninja: ARMClang6 response files require unix paths on Windows. Reference: https://gitlab.kitware.com/cmake/cmake/-/issues/21093
+
+## How to build an application
+
+Prerequisities:
+- CMake >=3.18.2
+- mbed-tools >=3.2.0
+
+From the application root or wherever `mbed-os.lib` is found:
+1. Run the following command to create the Mbed OS configuration CMake module: 
+
+    ```
+    mbedtools configure -m <mbed-target> -t <toolchain>
+    ```
+
+1. Run the following command to create a build directory, generate the project configuration and build the project using `Ninja`: 
+
+    ```
+    mkdir build && cd build && cmake .. -GNinja && cmake --build .
+    ```
+
+The default build type is `Develop`. Use `CMAKE_BUILD_TYPE` to select `Develop`, `Release` or `Debug` as follows: 
+
+```
+cmake .. -GNinja -DCMAKE_BUILD_TYPE=<supported-build-type> && cmake --build .
+```